Continuous delivery blood pumps used in heart assist applications have inherent size advantages over cyclic delivery blood pumps because they pump all the time and can operate at high speeds. For external temporary assist applications, the cannula size is reduced due to the continuous delivery feature. For implantable pumps, the smaller size of the pump itself eases anatomic placement constraints and allows placements which would otherwise be impossible. Continuous delivery blood pumps do, however, have certain disadvantages. The shear forces created by the relative motion of the impeller and housing may cause hemolysis (i.e. destruction of red cell membranes). Blood elements caught in eddies under the impeller shroud, or stagnating in unswept recesses, are prone to cause accumulations which, among other adverse effects, can produce thrombosis (i.e. the formation of dangerous blood clots). The use of anticoagulants to overcome this problem is not always medically advisable.
Conventional continuous delivery blood pumps have also exhibited serious problems of reliability due to clogging and eventual seizure of the bearings from blood elements entering, and accumulating in, the delicate bearings. Attempts have been made in the prior art (see U.S. Pat. Nos. 4,135,253 to Reich and 3,608,088 to Dorman) to reduce clogging by using hydrodynamic bearings, but any frictional shaft seal contains (or sooner or later develops) small recesses or crevices in which blood elements can become caught to initiate clotting and clogging. Although careful design of the impeller, housing and bearings can reduce its impact, thrombogenicity inherently exists whereever a rubbing interface is present in the blood pump between a rotating part and a stationary part.